First, verify actual coolant temp with an IR non-contact thermometer, or a good old kitchen thermometer. If gauge is indeed reading low, I can think of two possible causes:
1) the needle got moved somehow. Yes, you can remove the bezel and glass and re-position the needle--it's press-fit on a tiny spindle--but it'll be trial-and-error and you risk damaging a fragile mechanism, or
2) you lost some of the gas (ether) in the bulb, line and mechanism; though, I think this is unlikely because any leak would release all the gas (ether is HIGHLY flammable, so be careful if you do suspect a leak)
You'll probably need to get it overhauled; not cheap but it's important for the 'safety gauge' to be accurate. My oil pressure gauge used to indicate zero at hot, slow idle--obviously, I had oil pressure or the engine would have seized--but after overhaul I got 20PSI.